# Definition for a binary tree node.
class TreeNode:
  def __init__(self, x):
    self.val = x
    self.left = None
    self.right = None


class Solution:
  def maxDepth(self, root: TreeNode) -> int:
    if root is None:
      return 0
    else:
      return max(self.maxDepth(root.left), self.maxDepth(root.right)) + 1
